Piezoelectric crystal



, Feb. 6, 1962 R. BECHMANN 3,020,424

PIEZOELECTRIC CRYSTAL Filed May s, 1958 FIG. 5

DEClBELS I0.00 l0. IO 10.20 [0.30 IOAO FREQUENCY M. C.

DECIBELS IN VEN TOR,

' RUDOLF BECHMA NN.

1015 MC 0.5 1.6% 1.5% |.75% did;

FREQUENCY A T TORN )f United States Patent 3,020,424 PIEZOELECTRIC CRYSTAL Rudolf Bechmann, Oceanport, NJ assignor to the United States .of America .as represented by the Secretary of the Army Filed May 8, 1958, Ser. No. 734,062 1 Claim. (Cl. 310-95) (Granted under Title 35, US. Code (1952), scc. 26 6) The invention described hereinmay be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

This invention relates to the piezoelectric crystals which are operat ve at very high frequencies and possessgthe ability to overcome tendencies to oscillate at near harmonic frequencies. A

The invention'provides a solution to efficient operation of quartz crystals of the-AT and BI types when used in applications demanding very high oscillation frequency in a single mode and the ability to suppress other modes. Crystals having the qualities to meet the above demands, in effective degree, have not heretofore been available. Piezoelectric elements designed according to the principles of the invention exhibit marked improvement over existing crystal types,

The crystals referred to herein are those which are vibrated in thickness shear mode. Such crystals have been made in which spurious modes were suppressed to a limited degree. In developing these early techniques it was found that rectangular crystal plates failed to respond satisfactorily to rejection of spurious modes. Improved results werev obtained with peripherally round plates. By suitable bevelling of the edges of the plates a degree of spurious mode rejection could be established, but a great deal of improvement was desired. The most satisfactory crystals then used were round parallel faced plates having a diameter to thickness ratio less than 20. Crystals having slightly higher ratios showed some response to edge bevelling. However, in crystals having ratios 40 to 100 no satisfactory solution to controlling spurious modes was known. This was unfortunate because in the future more and more use will be made of the higher frequencies such as 10 to 30 megacycles.

The matter of physical size of these high frequency crystals has presented difficulties in their eflicient design. If they are too small the application of electrodes becomes more and more difiicult. For example, a 10 me. crystal vibrating in the fundamental mode has a diameter of 10 millimeters and a thickness of .166 millimeter- In such a crystal the practical minimum size for an electrode is 4 millimeters. Smaller electrodes are not desirable. The diameter thicknessratio in this crystal becomes about 60, which is its practical limit. A large diameter therefore cannot be used. Crystals for higher frequencies become very small and very thin. It was also found that existing techniques were ineffective in controlling spurious modes in such thin crystal plates. 7

It is a primary object of the invention to provide a piezoelectric crystal which is capable of suppressing unwanted modes of oscillation therein.

A further object of the invention is to provide a crystal which is particularly effective in discriminating against spurious modes at high frequencies such as at 10 me. or

more.

A further object of the invention is to provide a high frequency crystal having suitable discriminating characteristics and also providing adequate facility for applying electrodes thereto and for mounting the crystal.

A further object of the invention is to provide a crystal having a general conformation invoking special peripheral 3,020,424 Patented Feb. 6, 1962 contours adaptable 'to provide optimum discrimination against spurious modes.

Other objects and features of the invention will more fully appear from the following description and will 'be particularly pointed out in the claim.

To provide a better understanding qf the, invention specific formsthereof will be described and illustrated in the accompanying drawings wherein:

FIGURE 1 is a general view of a crystal prepared in accordance with the invention.

'FIGURES'Z, 3 and 4 are modified forms of thecrystal plate shown in FIGURE 1.

FIGURES -5 and 6 are respectively graphical representationsof performance of crystals constructed according tobestknowntechniques before the present invention and the performance of crystals *made according to the invention. y

The techniques of the present invention are directed to the control of spurious modes by designing the periph eral shape of the plates. Following the principles of the invention certain shapes have been developed which demonstrate the true capabilities of the invention. These shapes depart deliberately from rectangular and round configurations. As shown in the drawings the forms are generally polygonal but non rectangular. A very satisfactory conformation has been found to be the triangle.

Referring to the drawings, FIGURE 1 illustrates a straight sided triangular crystal 10 wherein its sides are formed by removing segments of a round crystaL. Its apexes 13 are designed to receive the electrodes 11 and 12. Its faces are ground and finished parallel or may be given a slightly convex curvature. t

The crystal 10 is designed for high frequency operation, its fundamental or overtone operating frequency being in the order of 10 megacycles or higher. This crystal has a diameter to thickness ratio of about 60 and functions in thickness shear mode. A suitable mounting is provided by soldered or spring clip connections to its electrodes.

The electrodes 11 and 12 are applied by plating on the respective faces of the crystal and are so disposed that they do not lie parallel to a diametrical line. A suitable arrangement is shown in FIGURE 1 wherein the electrodes are so placed that their inner ends are at the geometrical center of the crystal and extend radially outward to the outer edge of adjacent apex portions 13 of the crystal. Suitable diameters for high frequency crystals are 13 to 6 millimeters, whilethe useful electrode diameters are 4 to 2 millimeters.

Particular variations of the crystal shown in FIGURE 1 are those shown in FIGURES 2, 3 and 4. v In FIGURE 2, the sides 14 of the triangle are slightly convexed and in FIGURE 3 the sides 15 are concaved. While the straight sided shape is preferred, the other shapes are found to be effective in suppressing reflections causing standing; waves in the plane of the crystal. The segments removed from the round plate may be unsymmetric and of different size. The shapeshown in FIGURE-4 has been found to be highly efficient. In this construction the crystal is a true equilateral triangle having straight sides 16.

Actual tests show a great improvement in the performance of a crystal embodying the invention especially with respect to discrimination against unwanted frequencies. FIGURE 5 of the drawings illustrates graphically the attenuation versus frequency of a circular crystal having a fundamental frequency of 10 megacycles. This crystal was prepared according to methods existing before the present invention for avoiding spurious harmonics of vibration in the thickness shear mode. a The curve in this case shows undesirable response in modes relatively close to the fundamental peak response.

FIGURE 6 shows the response characteristic of a crystal having the same fundamental as that used in the test of FIGURE 5, but having the shape of the crystal shown in FIGURE 4. This test shows that the crystal in FIG- URE 4 substantially reduces spurious modes. in this latter example the first spurious mode from the wanted mode is about 1.5%, While the amplitude of the first spurious mode is down 60 decibels. The other forms of the crystals shown exhibit similar favorable characteristics.

What is, claimed is:

A thickness shear piezoelectric crystal element comprising a quartz plate having a peripheral contour in the shape of an equilateral triangle, said contour being derived from and having its apices extending substantially to the pcriphery of a disc shaped quartz crystal having a diameter to thickness ratio of about 60, said thickness and said equilateral triangle shape of said crystal being such as to restrict the crystal to oscillation in thickness shear mode at not less than 10 megacycles, and said oscillation occurring with a sharp peak response at its fundamental frequency and nearly complete rejection of spurious harmonies occurring close to the fundamentaifrequency.

References Cited in the file of this patent UNITED STATES PATENTS Re. 20,213 Sawyer Dec. 22, 1936 2,170,318 Cady Aug. 22, 1939 2,249,933 Bechmann July 22, 1941 2,343,059 Hight Feb. 29, 1944 2,485,722 Erwin Oct. 25, 1949 2,743,144 Bottom et a1 Apr. 24, 1956 2,967,958 Kosowsky et a1 Jan. 10, 1961 FOREIGN PATENTS 871,625 Germany Mar. 23, 1953 20 Book Co., New York. 

1. A THICKNESS SHEAR PIEZOELECTRIC CRYSTAL ELEMENT COMPRISING A QUARTZ PLATE HAVING A PERIPHERAL CONTOUR IN THE SHAPE OF AN EQUILATERAL TRIANGLE, SAID CONTOUR BEING DERIVED FROM AND HAVING ITS APICES EXTENDING SUBSTANTIALLY TO THE PERIPHERY OF A DISC SHAPED QUARTZ CRYSTAL HAVING A DIAMETER TO THICKNESS RATIO OF ABOUT 60, SAID THICKNESS AND SAID EQUILATERAL TRIANGLE SHAPE OF SAID CRYSTAL BEING SUCH AS TO RESTRICT THE CRYSTAL TO OSCILLATION IN THICKNESS SHEAR MODE AT NOT LESS THAN 10 MEGACYLES, AND SAID OSCILLATION OCCURRING WITH A SHARP PEAK RESPONSE AT ITS FUNDAMENTAL FREQUENCY AND NEARLY COMPLETE REJECTION OF SPURIOUS HARMONIES OCCURING CLOSE TO THE FUNDAMENTAL FREQUENCY. 